The present invention relates generally to coplanar waveguide, quadrature hybrids and is specifically designed to eliminate unwanted electromagnetic couplings between the ground planes of such devices.
In the article "Coplanar Waveguide: A Surface Strip Transmission Line Suitable for Nonreciprocal Gyromagnetic Device Applications", IEEE MTT, December 1969 (MTT-17, #12) pg. 1087 ff., C. P. Wen first proposed the basic coplanar waveguide coupled-line configuration as shown in FIGS. 1A and 1B. In FIG. 1B, coplanar waveguide (CPW) 10 comprises a first ground plane 1, coupled transmission lines 2 and 3, and a second ground plane 4, all of which are printed upon the same surface 6 of a dielectric substrate 5. The CPW lines of FIG. 1B can operate either in the even mode (i.e. the voltages and currents of one of the transmission elements are in phase with those of the other transmission element) or in the odd mode (i.e. the voltages and currents of one of the transmission elements are 180.degree. out of phase with those of the other transmission element). The length of the coupled (or parallel) sections of the transmission elements as shown in FIG. 1A is chosen to be 90 electrical degrees (one quarter wavelength) at the operating frequency.
Referring back to FIG. 1B, note that the evenmode impedance Zoe, the odd-mode impedance Zoo, and the coupling coefficients between the coupled sections of the transmitters, can all be altered by changing any of the length parameters a, b or c, where length "a" is the distance between the center line of transmission 20 and the abutting edge of a coupling element, length "b" is the distance between the center line of transmission and the non-abutting edge of a coupling element, and length "c" is the distance between the center line of transmission and the abutting edge of a ground plane.
The CPW device as described above is well suited for a variety of microstrip waveguide device applications. However, when the CPW of FIGS. 1A and 1B is excited in the odd mode, spurious electric fields are propagated between the ground planes of the device. With reference to FIG. 2, when the device of FIGS. 1A and 1B operates in the odd mode (i.e. the instantaneous voltages and currents in the coupling sections are of opposite sign), the naturally occurring electric field (as shown by the solid arrows) is not symmetrical about the center line of transmission 20. Accordingly, a spurious E-field (as shown by the dashed arrow) develops between the ground planes 1 and 4, producing a potential difference between the ground planes. This spurious "slot-line" propagation mode operates differently then the normal CPW propagation mode, and such a multi-moded operation is extremely detrimental to the operation of the coupler. Specifically, this spurious propagation mode makes CPW design difficult, since analysis of the performance of the coupler can only be undertaken assuming that all propagation takes place in a single mode, requiring both ground planes to be at the same potential.